Open Cydia, look for and install PushTone. Once you have PushTone installed you can assign a tone to Facebook. If you want to take it one step farther and use the actual alert that the FB app uses, then open iFile and go to the FB app. Inside there you will find a wav file called chord.wav. Use Bridge to import/convert that file as a ringtone (or whatever way you want to do this, but it has to be converted to a ringtone). Once you have it converted as a ringtone and installed you can tell PushTone to use "Chord" for it's alert sound and voila! You have the FB sound notification back.